Semiconductor device and method for manufacturing the same

ABSTRACT

In order to form a structure in which an oxide semiconductor layer through which a carrier flows is not in contact with a gate insulating film, a buried channel structure in which the oxide semiconductor layer through which a carrier flows is away from the gate insulating film containing silicon is provided. Specifically, a buffer layer is provided between the gate insulating film and the oxide semiconductor layer. Both the oxide semiconductor layer and the buffer layer are formed using materials containing indium and another metal element. The composition of indium with respect to gallium contained in the oxide semiconductor layer is higher than the composition of indium with respect to gallium contained in the buffer layer. The buffer layer has a smaller thickness than the oxide semiconductor layer.

TECHNICAL FIELD

The present invention relates to a semiconductor device including anoxide semiconductor and a manufacturing method thereof.

In this specification, a semiconductor device generally means a devicewhich can function by utilizing semiconductor characteristics, and anelectrooptic device, a semiconductor circuit, and electronic equipmentare all semiconductor devices.

BACKGROUND ART

In recent years, semiconductor devices have been developed to be usedmainly for an LSI, a CPU, or a memory. A CPU is an aggregation ofsemiconductor elements each provided with an electrode which is aconnection terminal, which includes a semiconductor integrated circuit(including at least a transistor and a memory) separated from asemiconductor wafer.

A semiconductor circuit (IC chip) of an LSI, a CPU, or a memory ismounted on a circuit board, for example, a printed wiring board, to beused as one of components of a variety of electronic devices.

A technique for manufacturing a transistor by using an oxidesemiconductor film for a channel formation region, or the like has beenattracting attention. Examples of such a transistor include a transistorin which zinc oxide (ZnO) is used as an oxide semiconductor film and atransistor in which InGaO₃(ZnO)_(m) is used as an oxide semiconductorfilm.

Patent Document 1 discloses a three-layer structure in which a firstmulti-component oxide semiconductor layer over a substrate, asingle-component oxide semiconductor layer over the firstmulti-component oxide semiconductor layer, and a second multi-componentoxide semiconductor layer over the single-component oxide semiconductorlayer are stacked.

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2011-155249

DISCLOSURE OF INVENTION

Electric characteristics of a transistor including an oxidesemiconductor layer are affected by an insulating film in contact withthe oxide semiconductor, that is, by an interface state between theoxide semiconductor layer and the insulating film.

For example, in the case where an insulating film containing silicon(e.g., silicon oxide film) is used as the insulating film and an oxidesemiconductor layer is formed by a sputtering method over the siliconoxide film, silicon might possibly enter the oxide semiconductor layerin sputtering. The entry of silicon into the oxide semiconductor layermay possibly cause a reduction in the field-effect mobility of atransistor.

Further, when a silicon nitride film is used as the insulating film, alarge amount of carriers flow in an interface between the siliconnitride film and the oxide semiconductor layer, which makes it difficultto obtain transistor characteristics.

One object is to provide a transistor structure with high filed-effectmobility.

Thus, a structure in which an oxide semiconductor layer through which acarrier flows is not in contact with a gate insulating film containingsilicon is formed. In order to achieve such a structure, a stacked-layerstructure in which a channel layer is sandwiched between the otherlayers (here, the structure is called “buried channel structure”) isprovided. In the buried channel structure, an oxide semiconductor layerthrough which a carrier flows is away from a gate insulating filmcontaining silicon. Specifically, a buffer layer is provided between thegate insulating film and the oxide semiconductor layer. Both the oxidesemiconductor layer and the buffer layer are formed using materialscontaining indium and another metal element (referred to as a metalelement M). Examples of the metal element M include gallium and hafnium.The composition of indium with respect to the metal element M (e.g.,gallium) in the oxide semiconductor layer is higher than the compositionof indium with respect to gallium in the buffer layer. Further, thebuffer layer with a thickness smaller than that of the oxidesemiconductor layer is formed using a material in which the compositionof indium with respect to the metal element in the oxide semiconductorlayer is low.

Further, it is preferable that the oxide semiconductor layer besandwiched between a first buffer layer and a second buffer layer so asto prevent the oxide semiconductor layer through which a carrier flowsfrom being in contact with the insulating film containing silicon.

A structure of the present invention disclosed in this specification isa semiconductor device including a first insulating layer over aninsulating surface, a first buffer layer over the first insulatinglayer, an oxide semiconductor layer over the first buffer layer, asecond buffer layer over the oxide semiconductor layer, and a secondinsulating layer over the second buffer layer. The oxide semiconductorlayer, the first buffer layer, and the second buffer layer are formedusing an oxide semiconductor material containing at least indium andgallium. A composition of indium with respect to gallium contained inthe oxide semiconductor layer is higher than a composition of indiumwith respect to gallium contained in the first and second buffer layers.The thickness of the oxide semiconductor layer is larger than those ofthe first and second buffer layers.

In the case of a bottom-gate transistor, a gate electrode layer isprovided between the insulating surface and the first insulating layerin the above structure.

On the other hand, in the case of a top-gate transistor, a gateelectrode layer is provided over the second insulating layer in theabove structure.

Further, in the case of a dual-gate transistor including gate electrodelayers over and below the oxide semiconductor layer, a first gateelectrode layer is provided between the insulating surface and the firstinsulating layer, and a second gate electrode layer is provided over thesecond insulating layer in the above structure.

A structure of an oxide semiconductor film is described below.

An oxide semiconductor film is classified roughly into a single-crystaloxide semiconductor film and a non-single-crystal oxide semiconductorfilm. The non-single-crystal oxide semiconductor film includes any of anamorphous oxide semiconductor film, a microcrystalline oxidesemiconductor film, a polycrystalline oxide semiconductor film, a c-axisaligned crystalline oxide semiconductor (CAAC-OS) film, and the like.

The amorphous oxide semiconductor film has disordered atomic arrangementand no crystalline component. A typical example thereof is an oxidesemiconductor film in which no crystal part exists even in a microscopicregion, and the whole of the film is amorphous.

The microcrystalline oxide semiconductor film includes a microcrystal(also referred to as nanocrystal) with a size greater than or equal to 1nm and less than 10 nm, for example. Thus, the microcrystalline oxidesemiconductor film has a higher degree of atomic order than theamorphous oxide semiconductor film. Hence, the density of defect statesof the microcrystalline oxide semiconductor film is lower than that ofthe amorphous oxide semiconductor film.

The CAAC-OS film is one of oxide semiconductor films including aplurality of crystal parts, and most of the crystal parts each fitsinside a cube whose one side is less than 100 nm. Thus, there is a casewhere a crystal part included in the CAAC-OS film fits inside a cubewhose one side is less than 10 nm, less than 5 nm, or less than 3 nm.The density of defect states of the CAAC-OS film is lower than that ofthe microcrystalline oxide semiconductor film. The CAAC-OS film isdescribed in detail below.

In a transmission electron microscope (TEM) image of the CAAC-OS film, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflected by a surface over which theCAAC-OS film is formed (hereinafter, a surface over which the CAAC-OSfilm is formed is referred to as a formation surface) or a top surfaceof the CAAC-OS film, and is arranged in parallel to the formationsurface or the top surface of the CAAC-OS film.

On the other hand, according to the TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface (plan TEM image), metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

From the results of the cross-sectional TEM image and the plan TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray enters a sample in a direction substantiallyperpendicular to the c-axis, a peak appears frequently when 2θ is around56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal.Here, analysis (φ scan) is performed under conditions where the sampleis rotated around a normal vector of a sample surface as an axis (100axis) with 2θ fixed at around 56°. In the case where the sample is asingle-crystal oxide semiconductor film of InGaZnO₄, six peaks appear.The six peaks are derived from crystal planes equivalent to the (110)plane. On the other hand, in the case of a CAAC-OS film, a peak is notclearly observed even when φ scan is performed with 2θ fixed at around56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallel toa normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer arranged in a layered mannerobserved in the cross-sectional TEM image corresponds to a planeparallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned in adirection parallel to a normal vector of a formation surface or a normalvector of a top surface. Thus, for example, in the case where a shape ofthe CAAC-OS film is changed by etching or the like, the c-axis might notbe necessarily parallel to a normal vector of a formation surface or anormal vector of a top surface of the CAAC-OS film.

Further, the degree of crystallinity in the CAAC-OS film is notnecessarily uniform. For example, in the case where crystal growthleading to the CAAC-OS film occurs from the vicinity of the top surfaceof the film, the degree of the crystallinity in the vicinity of the topsurface is higher than that in the vicinity of the formation surface insome cases. Further, when an impurity is added to the CAAC-OS film, thecrystallinity in a region to which the impurity is added is changed, andthe degree of crystallinity in the CAAC-OS film varies depending onregions.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ do not appear at around36°.

In a transistor using the CAAC-OS film, change in electriccharacteristics due to irradiation with visible light or ultravioletlight is small. Thus, the transistor has high reliability.

Note that an oxide semiconductor film may be a stacked film includingtwo or more films of an amorphous oxide semiconductor film, amicrocrystalline oxide semiconductor film, and a CAAC-OS film, forexample.

In this specification, a term “parallel” indicates that the angle formedbetween two straight lines is greater than or equal to −10° and lessthan or equal to 10°, and accordingly also includes the case where theangle is greater than or equal to −5° and less than or equal to 5°. Inaddition, a term “perpendicular” indicates that the angle formed betweentwo straight lines is greater than or equal to 80° and less than orequal to 100°, and accordingly includes the case where the angle isgreater than or equal to 85° and less than or equal to 95°.

In this specification, the trigonal and rhombohedral crystal systems areincluded in the hexagonal crystal system.

For example, the CAAC-OS film is formed by a sputtering method with apolycrystalline oxide semiconductor sputtering target. When ions collidewith the sputtering target, a crystal region included in the sputteringtarget may be separated from the target along an a-b plane; in otherwords, a sputtered particle having a plane parallel to an a-b plane(flat-plate-like sputtered particle or pellet-like sputtered particle)may flake off from the sputtering target. In that case, theflat-plate-like sputtered particle reaches a substrate while keeping itscrystal state, whereby the crystal state of the sputtering target istransferred to the substrate and the CAAC-OS film can be formed.

For the deposition of the CAAC-OS film, the following conditions arepreferably used.

Reduction in the amount of impurities entering the CAAC-OS film duringthe deposition can prevent the crystal state from being broken by theimpurities. For example, impurities (e.g., hydrogen, water, carbondioxide, or nitrogen) which exist in a deposition chamber may bereduced. Furthermore, impurities in a deposition gas may be reduced.Specifically, a deposition gas whose dew point is −80° C. or lower,preferably −100° C. or lower is used.

By increasing the substrate heating temperature during the deposition,migration of a sputtered particle is likely to occur after the sputteredparticle is attached to a substrate surface. Specifically, the substrateheating temperature during the deposition is higher than or equal to100° C. and lower than or equal to 740° C., preferably higher than orequal to 200° C. and lower than or equal to 500° C. By increasing thesubstrate heating temperature during the deposition, when theflat-plate-like sputtered particle reaches the substrate, migrationoccurs on the substrate surface, so that a flat plane of theflat-plate-like sputtered particle is attached to the substrate.

Furthermore, it is preferable that the proportion of oxygen in thedeposition gas be increased and the power be optimized in order toreduce plasma damage at the deposition. The proportion of oxygen in thedeposition gas is 30 vol. % or higher, preferably 100 vol. %.

As an example of the sputtering target, an In—Ga—Zn—O compound target isdescribed below.

The In—Ga—Zn—O compound target, which is polycrystalline, is made bymixing InO_(X) powder, GaO_(Y) powder, and ZnO_(Z) powder in apredetermined ratio, applying pressure, and performing heat treatment ata temperature higher than or equal to 1000° C. and lower than or equalto 1500° C. Note that X, Y, and Z are each a given positive number.Here, the predetermined molar ratio of InO_(X) powder to GaO_(Y) powderand ZnO_(Z) powder is, for example, 2:2:1, 8:4:3, 3:1:1, 1:1:1, 4:2:3,or 3:1:2. The kinds of powder and the molar ratio for mixing powder maybe determined as appropriate depending on the desired sputtering target.

In a transistor using the CAAC-OS film, change in electriccharacteristics of the transistor due to irradiation with visible lightor ultraviolet light is small. Thus, the transistor has highreliability.

In the case where an oxide semiconductor layer and a buffer layer areCAAC-OS films, the oxide semiconductor layer and the buffer layer havethe same crystal structure; thus, there are few defects at theinterface, and high field-effect mobility can be achieved. Further, itis preferable that the buffer layer be formed over and in contact withthe oxide semiconductor layer that is a CAAC-OS film. This is becausethe oxide semiconductor layer serves as a seed of crystal, and thebuffer layer formed thereover is easily crystallized to have the samecrystal structure as the oxide semiconductor layer.

A carrier flow path is generated at about 5 nm away from an interface ofthe gate insulating layer; thus, the thickness of the buffer layer isgreater than or equal to 2 nm and less than or equal to 15 nm,preferably greater than or equal to 5 nm and less than or equal to 10nm. The oxide semiconductor layer is formed thicker than the bufferlayer. In such a structure, a carrier flows at the interface between thebuffer layer and the oxide semiconductor layer or in the oxidesemiconductor layer. In other words, in such a structure, the oxidesemiconductor layer through which a carrier flows is away from the gateinsulating film containing silicon.

A transistor structure with high field-effect mobility can be achieved.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1D are cross-sectional views illustrating manufacturingsteps of one embodiment of the present invention.

FIGS. 2A to 2C are respectively a cross-sectional view, a top view, andan energy band diagram illustrating one embodiment of the presentinvention.

FIGS. 3A and 3B are cross-sectional views illustrating one embodiment ofthe present invention.

FIGS. 4A and 4B are a cross-sectional view and a circuit diagramillustrating one embodiment of a semiconductor device.

FIGS. 5A to 5C are a cross-sectional view and circuit diagramsillustrating one embodiment of a semiconductor device.

FIG. 6 is a circuit diagram illustrating one embodiment of asemiconductor device.

FIG. 7 is a perspective view illustrating one embodiment of asemiconductor device.

FIGS. 8A to 8C are a block diagram and circuit diagrams illustrating oneembodiment of a semiconductor device.

FIGS. 9A to 9C illustrate an electronic device.

FIGS. 10A to 10C illustrate an electronic device.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that modes and details disclosedherein can be modified in various ways without departing from the spiritand the scope of the present invention. Therefore, the present inventionis not construed as being limited to description of the embodiments.

Embodiment 1

In this embodiment, one embodiment of a semiconductor device and amethod for manufacturing the semiconductor device will be described withreference to FIGS. 1A to 1D. In this embodiment, an example of a methodfor manufacturing a transistor including an oxide semiconductor film isdescribed.

First, an insulating film 433 is formed over a substrate 400 having aninsulating surface. A conductive film is formed over the insulating film433 by a sputtering method, an evaporation method, or the like, and theconductive film is etched, so that a conductive layer 491 and wiringlayers 434 and 436 are formed.

There is no particular limitation on a substrate that can be used as thesubstrate 400 having an insulating surface as long as it has heatresistance high enough to withstand heat treatment performed later. Forexample, a glass substrate of barium borosilicate glass,aluminoborosilicate glass, or the like, a ceramic substrate, a quartzsubstrate, or a sapphire substrate can be used. A single crystalsemiconductor substrate or a polycrystalline semiconductor substrate ofsilicon, silicon carbide, or the like; a compound semiconductorsubstrate of silicon germanium or the like; an SOI substrate; or thelike can be used as the substrate 400, or the substrate provided with asemiconductor element can be used as the substrate 400.

The insulating film 433 can be formed using one or more insulating filmsselected from the following: an oxide insulating film of silicon oxide,gallium oxide, hafnium oxide, yttrium oxide, aluminum oxide, or thelike; a nitride insulating film of silicon nitride, aluminum nitride, orthe like; an oxynitride insulating film of silicon oxynitride, aluminumoxynitride, or the like; or a nitride oxide insulating film of siliconnitride oxide or the like. Note that “silicon nitride oxide” containsmore nitrogen than oxygen and “silicon oxynitride” contains more oxygenthan nitrogen. In the case where a substrate provided with asemiconductor element is used, a silicon nitride film is preferably usedas the insulating film 433, which is formed by a plasma CVD method withuse of a mixed gas of silane (SiH₄) and nitrogen (N₂) as a supply gas.This silicon nitride film also functions as a barrier film, which has afunction of preventing entry of hydrogen or a hydrogen compound into anoxide semiconductor layer formed later so as to improve the reliabilityof a semiconductor device. In the case where the silicon nitride film isformed by a plasma chemical vapor deposition (CVD) method with use of amixed gas of silane (SiH₄), ammonia (NH₃), and nitrogen (N₂) as a supplygas, the amount of defects in the film can be reduced as compared withthe case where the silicon nitride film is formed with use of a mixedgas of silane (SiH₄) and nitrogen (N₂) as a supply gas. When thethickness of the silicon nitride film formed with use of a mixed gas ofsilane (SiH₄), ammonia (NH₃), and nitrogen (N₂) as a supply gas isgreater than or equal to 300 nm and less than or equal to 400 nm, theESD resistance can be 300 V or higher. Thus, in the case where theinsulating film 433 is a stacked film in which a silicon nitride film isformed with use of a mixed gas of silane (SiH₄), ammonia (NH₃), andnitrogen (N₂) to have a thickness greater than or equal to 300 nm andless than or equal to 400 nm and a silicon nitride film is formedthereover with use of a mixed gas of silane (SiH₄) and nitrogen (N₂) asa supply gas, a barrier film with high ESD resistance can be obtained.

The conductive layer 491 and the wiring layers 434 and 436 can be formedusing a metal material such as molybdenum, titanium, tantalum, tungsten,aluminum, copper, chromium, neodymium, or scandium or an alloy materialwhich contains any of these materials as its main component.Alternatively, a semiconductor film typified by a polycrystallinesilicon film doped with an impurity element such as phosphorus, or asilicide film such as a nickel silicide film may be used as theconductive layer 491 and the wiring layers 434 and 436. The conductivelayer 491 and the wiring layers 434 and 436 may have a single-layerstructure or a stacked structure.

The conductive layer 491 and the wiring layers 434 and 436 can also beformed using a conductive material such as indium oxide-tin oxide,indium oxide containing tungsten oxide, indium zinc oxide containingtungsten oxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium oxide-zinc oxide, or indium tin oxideto which silicon oxide is added. It is also possible that the conductivelayer 491 and the wiring layers 434 and 436 have a stacked structure ofthe above conductive material and the above metal material.

In order to obtain a normally-off switching element, it is preferablethat the threshold voltage of the transistor is made positive by using amaterial having a work function of 5 eV (electron volts) or higher,preferably 5.5 eV or higher, for a gate electrode layer. Specifically, amaterial which includes an In—N bond and has a specific resistivity of1×10⁻¹ Ω.cm to 1×10⁻⁴ Ω.cm, preferably 5×10⁻² Ω.cm to 1×10⁻⁴ Ω.cm, isused for the gate electrode layer. Examples of the material are anIn—Ga—Zn-based oxide film containing nitrogen, an In—Sn—O filmcontaining nitrogen, an In—Ga—O film containing nitrogen, an In—Zn—Ofilm containing nitrogen, an In—O film containing nitrogen, and a metalnitride film (e.g., an InN film).

Next, an oxide insulating film is formed over the conductive layer 491and the wiring layers 434 and 436. The oxide insulating film hasprojections caused by the shape of the conductive layer 491 and thewiring layers 434 and 436 on its surface.

The oxide insulating film can be formed by a plasma CVD method, asputtering method, or the like using any of silicon oxide, siliconoxynitride, aluminum oxide, aluminum oxynitride, hafnium oxide, galliumoxide, gallium zinc oxide, and zinc oxide, or a mixed material thereof.The oxide insulating film may have either a single-layer structure or astacked structure.

Then, polishing treatment (e.g., chemical mechanical polishing: CMP) isperformed, so that an oxide insulating film 435 which is a planarizedfilm is formed and top surfaces of the wiring layers 434 and 436 and atop surface of the conductive layer 491 are exposed. After the CMPtreatment, cleaning is performed, and heat treatment for removingmoisture attached to the substrate is performed. A cross-sectional viewillustrating a state where steps up to and including the heat treatmenthave been done corresponds to FIG. 1A.

After the planarization, an insulating film 437 and a stacked layer 403including oxide semiconductor films are formed. A cross-sectional viewillustrating a state where the steps up to and including the formationhave been done corresponds to FIG. 1B.

Then, patterning is performed. Here, with use of one mask, theinsulating film 437 and the stacked layer 403 including oxidesemiconductor films are selectively etched. A cross-sectional viewillustrating a state where steps up to and including the etching havebeen done corresponds to FIG. 1C. It is preferable that the insulatingfilm 437 and the stacked layer 403 including oxide semiconductor filmsbe successively formed without being exposed to the air because impuritycontamination at the film interface can be prevented.

The insulating film 437 is formed by a plasma CVD method or a sputteringmethod. Among plasma CVD methods, the following plasma CVD method thatis also referred to as a microwave plasma CVD method is preferably used:plasma is generated by utilizing field effect energy of a microwaveparticularly, a source gas of the insulating film is excited by theplasma, the excited source gas is reacted over a formation subject, andthe reactant is deposited. Further, the insulating film formed by aplasma CVD method using a microwave is a dense film; thus, theinsulating film 437 formed by processing the insulating film is also adense film. The thickness of the insulating film 437 is greater than orequal to 5 nm and less than or equal to 300 nm.

The insulating film 437 can be formed using one or more insulating filmsselected from the following: an oxide insulating film of silicon oxide,gallium oxide, hafnium oxide, yttrium oxide, aluminum oxide, or thelike; an oxynitride insulating film of silicon oxynitride, aluminumoxynitride, or the like; or a nitride oxide insulating film of siliconnitride oxide or the like. As another material of the insulating film437, a target with an atomic ratio where In:Ga:Zn=1:3:2 may be used toform an In—Ga—Zn-based oxide film.

In this embodiment, the stacked layer 403 including oxide semiconductorfilms has a three-layer structure where a first oxide semiconductor film403 a, a second oxide semiconductor film 403 b, and a third oxidesemiconductor film 403 c are stacked in this order as illustrated inFIG. 1C.

The oxide semiconductor film can be formed using an oxide including atleast In and another metal element M (M is Ga, Hf, Zn, Mg, Sn, or thelike). Examples of such an oxide include the following oxides: atwo-component metal oxide such as an In—Zn-based oxide, an In—Mg-basedoxide, or an In—Ga-based oxide; a three-component metal oxide such as anIn—Ga—Zn-based oxide (also referred to as IGZO), an In—Sn—Zn-basedoxide, an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, anIn—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, In—Nd—Zn-based oxide, anIn—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide,an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-basedoxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, anIn—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide; and a four-componentmetal oxide such as an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-basedoxide, or an In—Sn—Hf—Zn-based oxide.

As the first oxide semiconductor film 403 a, a 10-nm-thickIn—Ga—Zn-based oxide film formed with a target having an atomic ratiowhere In:Ga:Zn=1:1:1 is used. Note that the first oxide semiconductorfilm 403 a can be referred to as a first buffer layer.

As the second oxide semiconductor film 403 b, a 20-nm-thickIn—Ga—Zn-based oxide film formed with a target having an atomic ratiowhere In:Ga:Zn=3:1:2 is used. The composition of indium with respect togallium contained in the second oxide semiconductor film 403 b is higherthan the composition of indium with respect to gallium contained in thefirst buffer layer. In the second oxide semiconductor film 403 b, theamount of indium is preferably larger than that of gallium.

As the third oxide semiconductor film 403 c, a 10-nm-thickIn—Ga—Zn-based oxide film formed with a target having an atomic ratiowhere In:Ga:Zn=1:1:1 is used. Note that the third oxide semiconductorfilm 403 c can be referred to as a second buffer layer.

The first buffer layer and the second buffer layer each have a smallerthickness than the second oxide semiconductor film 403 b through which acarrier flows. Further, as compared with a material of the second oxidesemiconductor film 403 b through which a carrier flows, a materialhaving a low composition of indium with respect to a metal elementcontained in the oxide semiconductor layer is used for the first bufferlayer and the second buffer layer. It is preferable that each of thefirst buffer layer and the second buffer layer have such a compositionthat the amount of indium is the same as or less than or equal to thatof gallium.

In such a stacked structure, the second oxide semiconductor film 403 bthrough which a carrier flows is not in contact with the insulating filmcontaining silicon.

Further, targets used for forming the first oxide semiconductor film 403a and the third oxide semiconductor film 403 c and a target used forforming the second oxide semiconductor film 403 b are preferablypolycrystalline targets, and CAAC-OS films are preferably formed as thefirst to third oxide semiconductor films. Alternatively, when the secondoxide semiconductor film 403 b has a composition which enables the filmto be easily crystallized, the first oxide semiconductor film 403 a andthe third oxide semiconductor film 403 c, which are in contact with thesecond oxide semiconductor film 403 b, can be crystallized. Since thereare few defects at the interface between the first oxide semiconductorfilm 403 a and the second oxide semiconductor film 403 b and few defectsat the interface between the second oxide semiconductor film 403 b andthe third oxide semiconductor film 403 c, high field-effect mobility canbe achieved. The thicknesses and compositions of the oxide semiconductorfilms are preferably adjusted so that a carrier flows through only thesecond oxide semiconductor film 403 b.

The insulating film 437 and the first oxide semiconductor film 403 a aresuccessively formed without being exposed to the air, which can preventthe interface between the insulating film 437 and the first oxidesemiconductor film 403 a from being contaminated by impurities. Thesecond oxide semiconductor film 403 b and the third oxide semiconductorfilm 403 c are successively formed without being exposed to the air,which can prevent the interface between the second oxide semiconductorfilm 403 b and the third oxide semiconductor film 403 c from beingcontaminated by impurities. In addition, the third oxide semiconductorfilm 403 c also functions as a protective film which prevents the secondoxide semiconductor film 403 b from being in contact with the air in alater step such as an etching step. Thus, impurities such as silicon areprevented from entering the inside of the second oxide semiconductorfilm 403 b through which a carrier flows and the interfaces of the film(on both the upper surface side and the bottom surface side of thefilm), which achieves high field-effect mobility.

After the insulating film 437 and the stacked layer 403 including oxidesemiconductor films are formed, a conductive film is formed. Thisconductive film is selectively etched, so that electrode layers 445 aand 445 b and a conductive layer 442 are formed. A cross-sectional viewillustrating a state where steps up to and including the formation havebeen done corresponds to FIG. 1D. By performing etching plural times atthis time, electrodes each having projections at lower end portions whenseen in cross section are formed. The electrode layers 445 a and 445 beach having projections at lower end portions function as source anddrain electrode layers of a transistor. The electrode layer 445 a isprovided over and in contact with the wiring layer 436. The electrodelayer 445 b is provided over and in contact with the wiring layer 434.

A distance between the electrode layers 445 a and 445 b corresponds to achannel length L of the transistor. In the case where the channel lengthL of the transistor is less than 50 nm, for example, about 30 nm, adeveloped mask which is obtained by exposing a resist with use of anelectron beam is preferably used as a mask for etching the conductivefilm. At a higher acceleration voltage, an electron beam can provide amore precise pattern. The use of multiple electron beams as an electronbeam can shorten the process time per substrate. Here, in an electronbeam writing apparatus capable of electron beam irradiation, theacceleration voltage is preferably in the range from 5 kV to 50 kV, forexample. The current intensity is preferably in the range from 5×10⁻¹² Ato 1×10⁻¹¹ A. The minimum beam size is preferably 2 nm or less. Theminimum possible pattern line width is preferably 8 nm or less. Underthe above conditions, a pattern with a width of, for example, 30 nm orless, preferably 20 nm or less, more preferably 8 nm or less, can beobtained.

Then, an insulating film 402 is formed over the electrode layers 445 aand 445 b, the conductive layer 442, and the stacked layer 403 includingoxide semiconductor films. A material of the insulating film 402 can bea silicon oxide film, a gallium oxide film, a gallium zinc oxide film, aGa₂O₃ (Gd₂O₃) film, a zinc oxide film, an aluminum oxide film, a siliconnitride film, a silicon oxynitride film, an aluminum oxynitride film, ora silicon nitride oxide film. As another material of the insulating film402, an In—Ga—Zn-based oxide film having low conductivity can be given.The In—Ga—Zn-based oxide film having low conductivity may be formedunder the following conditions: an oxide target containing In, Ga, andZn in an atomic ratio of 1:3:2 is used, the substrate temperature isroom temperature, and a sputtering gas is an argon gas or a mixed gas ofargon and oxygen.

The insulating film 402 preferably includes a region containing oxygenin excess of the stoichiometric composition (the region is referred toas an oxygen excess region). When an insulating layer in contact withthe stacked layer 403 including oxide semiconductor films includes anoxygen excess region, oxygen can be supplied to the stacked layer 403including oxide semiconductor films. Thus, elimination of oxygen fromthe stacked layer 403 can be prevented, and oxygen can be supplied tooxygen vacancies in the stacked layer 403. In order to provide theoxygen excess region in the insulating film 402, the insulating film 402may be formed in an oxygen atmosphere, for example. Alternatively,oxygen may be introduced into the formed insulating film 402 to providethe oxygen excess region therein. Further, the insulating film 402preferably has a stacked structure. Over an insulating film including anoxygen excess region, a silicon oxide film or a silicon oxynitride filmis formed under conditions where a high-frequency power that is higherthan or equal to 0.17 W/cm² and lower than or equal to 0.5 W/cm²,preferably, higher than or equal to 0.26 W/cm² and lower than or equalto 0.35 W/cm² is supplied. Specifically, the conditions for forming thesilicon oxynitride film are as follows: silane (SiH₄) and dinitrogenmonoxide (N₂O) which are source gases are supplied at 160 sccm and 4000sccm, respectively, into a treatment chamber; the pressure of thetreatment chamber is adjusted to 200 Pa; and a power of 1500 W issupplied with a high-frequency power supply of 27.12 MHz. Further, thesubstrate temperature at which the silicon oxynitride film is formed is220° C.

Next, the insulating film 402 is selectively etched to have an openingreaching the conductive layer 442. Then, a conductive film is formed andselectively etched, so that an electrode layer 438 electricallyconnected to the conductive layer 442 and a gate electrode layer 401overlapping with the stacked layer 403 including oxide semiconductorfilms with the insulating film 402 interposed therebetween are formed.An insulating film 407 functioning as a barrier film is provided so asto cover the gate electrode layer 401 and the electrode layer 438.

As the insulating film 407, a silicon nitride film is preferably formedby a plasma CVD method with use of a mixed gas of silane (SiH₄) andnitrogen (N₂) as a supply gas. This silicon nitride film functions as abarrier film, which has a function of preventing entry of hydrogen or ahydrogen compound into the oxide semiconductor films so as to improvethe reliability of a semiconductor device.

The gate electrode layer 401 and the electrode layer 438 can be formedusing a metal material such as molybdenum, titanium, tantalum, tungsten,aluminum, copper, chromium, neodymium, or scandium or an alloy materialwhich contains any of these materials as its main component. Asemiconductor film which is doped with an impurity element such asphosphorus and is typified by a polycrystalline silicon film, or asilicide film of nickel silicide or the like can also be used as thegate electrode layer 401 and the electrode layer 438. The gate electrodelayer 401 and the electrode layer 438 may have a single-layer structureor a stacked structure.

In this embodiment, a tungsten film is used as the gate electrode layer401 over and in contact with the insulating film 402.

Through the above steps, a transistor 415 of this embodiment can bemanufactured (see FIG. 2A). The transistor 415 is an example of adual-gate transistor. FIG. 2A is a cross-sectional view taken along thechannel length direction of the transistor 415. Note that in thedual-gate transistor 415, the insulating film 437 functions as a firstgate insulating film, and the insulating film 402 functions as a secondgate insulating film.

The conductive layer 491 can serve as a back gate which controls theelectrical characteristics of the transistor 415. For example, bysetting the potential of the conductive layer 491 to GND (or a fixedpotential), the threshold voltage of the transistor 415 is shifted in apositive direction, so that the transistor can be normally-off.

Further, when the conductive layer 491 is not provided, a top-gatetransistor can be obtained. Without a change in the number of steps,both a dual-gate transistor and a top-gate transistor can be formed overone substrate by changing a layout.

FIG. 2B shows an example of a top view of the transistor 415 whose crosssection taken along dotted line X-Y in FIG. 2B corresponds to FIG. 2A

Further, FIG. 2C is an energy band diagram in the thickness direction inFIG. 2A. In this embodiment, materials of the first oxide semiconductorfilm 403 a, the second oxide semiconductor film 403 b, and the thirdoxide semiconductor film 403 c are selected so that an energy band shownin FIG. 2C is obtained. Note that when a channel buried in thestacked-layer structure is formed in the conduction band, sufficienteffects can be obtained. Thus, an energy band diagram is not necessarilylimited to a structure in which both the conduction band and the valenceband have a depressed portion as in FIG. 2C; for example, a structure inwhich only the conduction band has a depressed portion may be employed.

Embodiment 2

In this embodiment, an example of a method for manufacturing abottom-gate transistor will be described below. Part of steps in amanufacturing process is the same as that in Embodiment 1, and thus thedetailed description of the part of the steps is omitted.

First, steps up to and including the step in FIG. 1C, which aredescribed in Embodiment 1, are performed. Specifically, over thesubstrate 400, the conductive layer 491 and the wiring layers 434 and436 are formed. Over the conductive layer 491 and the wiring layers 434and 436, an oxide insulating film is formed. Then, polishing treatment(CMP) is performed, so that the planarized oxide insulating film 435 isformed and top surfaces of the wiring layers 434 and 436 and a topsurface of the conductive layer 491 are exposed. After the CMPtreatment, cleaning is performed, and then heat treatment for removingmoisture attached on the substrate is performed. After theplanarization, the insulating film 437 and the stacked layer 403including oxide semiconductor films are formed. Then, patterning isperformed. Here, with use of one mask, the insulating film 437 and thestacked layer 403 including oxide semiconductor films are selectivelyetched. Through the steps up to here, the same stage as that in FIG. 1Cis obtained.

In this embodiment, with use of the resist mask which has been used inthe step of selectively etching the insulating film 437 and the stackedlayer 403 including oxide semiconductor films, the oxide insulating film435 is etched to be partly thinned, so that areas of the exposed topsurfaces of the wiring layers 434 and 436 are increased. Then, aconductive film is formed, and the conductive film is selectivelyetched, so that the electrode layers 445 a and 445 b and the conductivelayer 442 are formed.

After that, the insulating film 402 is formed over the electrode layers445 a and 445 b, the conductive layer 442, and the stacked layer 403including oxide semiconductor films.

Next, the insulating film 402 is selectively etched to have an openingreaching the conductive layer 442. Then, a conductive film is formed,and the conductive film is selectively etched to form the electrodelayer 438 that is electrically connected to the conductive layer 442.The insulating film 407 functioning as a bather film is provided so asto cover the electrode layer 438.

Through the above steps, a transistor 416 of this embodiment can bemanufactured (see FIG. 3A). The transistor 416 is an example of abottom-gate transistor. FIG. 3A is a cross-sectional view taken alongthe channel length direction of the transistor 416.

FIG. 3B illustrates another example of a bottom-gate transistor. In atransistor 417 illustrated in FIG. 3B, after the steps up to andincluding the step in FIG. 1C of Embodiment 1 are performed, aninterlayer insulating film 439 is provided, and an opening reaching thewiring layer 434 and an opening reaching the wiring layer 436 are formedin the interlayer insulating film 439. Then, the electrode layers 445 aand 445 b are formed. The electrode layer 445 a is electricallyconnected to the wiring layer 436, and the electrode layer 445 b iselectrically connected to the wiring layer 434.

Note that the interlayer insulating film 439 may be formed using thesame material as that in the insulating film 402.

Each of the transistor 416 in FIG. 3A and the transistor 417 in FIG. 3Bhas a structure such that yield can be improved by securely connectingthe electrode layers 445 a and 445 b to the wiring layers 434 and 436.

This embodiment can be freely combined with Embodiment 1.

Embodiment 3

In this embodiment, an example of a semiconductor device including thetransistor described in Embodiment 1 will be described with reference toFIGS. 4A and 4B.

The semiconductor device illustrated in FIGS. 4A and 4B includestransistors 740 and 750 including a first semiconductor material in alower portion, and a transistor 610 including a second semiconductormaterial in an upper portion. The transistor 610 has the same structureas the transistor 415 described in Embodiment 1. The same referencenumerals are used for the same parts as those in FIGS. 2A to 2C. FIG. 4Bis a circuit diagram of the semiconductor device in FIG. 4A.

Here, the first semiconductor material and the second semiconductormaterial are preferably materials having different band gaps. Forexample, the first semiconductor material may be a semiconductormaterial other than an oxide semiconductor (e.g., silicon) and thesecond semiconductor material may be an oxide semiconductor. Atransistor including a material such as silicon can easily operate athigh speed. On the other hand, a transistor including an oxidesemiconductor enables charge to be held for a long time owing to itscharacteristics.

As a substrate used in the semiconductor device, a single crystalsemiconductor substrate or a polycrystalline semiconductor substratemade of silicon or silicon carbide, a compound semiconductor substratemade of silicon germanium or the like, a silicon on insulator (SOI)substrate, or the like can be used. A channel formation region of thetransistor can be formed in or over the semiconductor substrate. Thesemiconductor device in FIG. 4A is an example in which the channelformation region is formed in the semiconductor substrate to form alower transistor.

In the semiconductor device in FIG. 4A, a single crystal siliconsubstrate is used as a substrate 700, and the transistors 740 and 750are formed using the single crystal silicon substrate. As the firstsemiconductor material, single crystal silicon is used. The transistor740 is an n-channel transistor and the transistor 750 is a p-channeltransistor. The transistors 740 and 750 are electrically connected toeach other to form a complementary metal oxide semiconductor (CMOS)circuit 760.

Note that in this embodiment, since the single crystal silicon substratehaving p-type conductivity is used as the substrate 700, an impurityelement imparting n-type conductivity is added to a formation region ofthe transistor 750 that is the p-channel transistor to form an n-well. Achannel formation region 753 of the transistor 750 is formed in then-well. As the impurity element imparting n-type conductivity,phosphorus (P), arsenic (As), or the like can be used.

Therefore, an impurity element imparting p-type conductivity is notadded to a formation region of the transistor 740 that is the n-channeltransistor; however, a p-well may be formed by adding an impurityelement imparting p-type conductivity. As the impurity element impartingp-type conductivity, boron (B), aluminum (Al), gallium (Ga), or the likemay be used.

On the other hand, in the case of using a single crystal siliconsubstrate having n-type conductivity, an impurity element impartingp-type conductivity may be added to form a p-well.

The transistor 740 includes a channel formation region 743, an n-typeimpurity region 744 functioning as a lightly doped drain (LDD) region oran extension region, an n-type impurity region 745 functioning as asource region or a drain region, a gate insulating film 742, and a gateelectrode layer 741. The n-type impurity region 745 has a higherimpurity concentration than the n-type impurity region 744. Sidewallinsulating layers 746 are provided on side surfaces of the gateelectrode layer 741. The n-type impurity regions 744 and the n-typeimpurity regions 745 having different impurity concentrations can beformed in a self-aligned manner by using the gate electrode layer 741and the sidewall insulating layers 746 as masks.

The transistor 750 includes the channel formation region 753, a p-typeimpurity region 754 functioning as a lightly doped drain (LDD) region oran extension region, a p-type impurity region 755 functioning as asource region or a drain region, a gate insulating film 752, and a gateelectrode layer 751. The p-type impurity region 755 has a higherimpurity concentration than the p-type impurity region 754. Sidewallinsulating layers 756 are provided on side surfaces of the gateelectrode layer 751. The p-type impurity regions 754 and the p-typeimpurity regions 755 having different impurity concentrations can beformed in a self-aligned manner by using the gate electrode layer 751and the sidewall insulating layers 756 as masks.

In the substrate 700, an element separation region 789 separates thetransistor 740 and the transistor 750, and insulating films 788 and 687are stacked over the transistor 740 and the transistor 750. A wiringlayer 647 electrically connected to the n-type impurity region 745through an opening in the insulating film 788 and the insulating film687 and a wiring layer 657 electrically connected to the p-type impurityregion 755 through an opening in the insulating film 788 and theinsulating film 687 are provided over the insulating film 687. A wiringlayer 748 is provided over the insulating film 687 so as to electricallyconnect the transistor 740 and the transistor 750. The wiring layer 748is electrically connected to the n-type impurity region 745 through anopening that is formed in the insulating film 788 and the insulatingfilm 687 and reaches the n-type impurity region 745. Further, the wiringlayer 748 is electrically connected to the p-type impurity region 755through an opening that is formed in the insulating film 788 and theinsulating film 687 and reaches the p-type impurity region 755.

An insulating film 686 is provided over the insulating film 687, thewiring layer 647, the wiring layer 748, and the wiring layer 657. Awiring layer 658 is formed over the insulating film 686. The wiringlayer 658 is electrically connected to a gate wiring through an openingin the insulating films 788, 687, and 686. The gate wiring is formedover the gate insulating film 742 and the gate insulating film 752. Thegate wiring branches into the gate electrode layer 741 and the gateelectrode layer 751.

The semiconductor device of this embodiment is not limited to thestructure in FIG. 4A. As the transistors 740 and 750, a transistorcontaining silicide (salicide) or a transistor which does not include asidewall insulating layer may be used. With a structure having asilicide (salicide), resistance of the source region and the drainregion can be lowered and the speed of the semiconductor device can beincreased. In addition, the semiconductor device can operate at lowvoltage, and thus the power consumption thereof can be reduced.

Next, the structures of upper elements provided over the lowertransistor in the semiconductor device in FIGS. 4A and 4B are described.

An insulating film 684 is stacked over the insulating film 686 and thewiring layer 658. The conductive layer 491, the wiring layer 434 and awiring layer 692 are formed over the insulating film 684.

The oxide insulating film 435 is provided so as to bury the conductivelayer 491, the wiring layer 434 and the wiring layer 692. Over the oxideinsulating film 435, the insulating film 437 is provided. Over theinsulating film 437, the first oxide semiconductor film 403 a, thesecond oxide semiconductor film 403 b having different composition fromthat of the first oxide semiconductor film 403 a, and the third oxidesemiconductor film 403 c having the almost same composition as the firstoxide semiconductor film 403 a are provided in this order. In addition,over the third oxide semiconductor film 403 c, the electrode layers 445a and 445 b each having projections at lower end portions are provided.The insulating film 402 is provided over and in contact with a region ofthe second oxide semiconductor film 403 b which overlaps with neitherthe source electrode layer 445 a nor the drain electrode layer 445 b(the region is a channel formation region). The gate electrode layer 401is provided over the insulating film 402.

A capacitor 690 is also formed over the oxide insulating film 435 by thesame process as that of the transistor 610. The capacitor 690 includesthe electrode layer 445 a functioning as one of electrodes, a capacitorelectrode layer 693 functioning as the other electrode, and theinsulating film 402 which is provided between the electrodes andfunctions as a dielectric. The capacitor electrode layer 693 is formedin the same step as the gate electrode layer 401.

By setting the potential of the conductive layer 491 to GND (or a fixedpotential), the conductive layer 491 serves as a back gate whichcontrols the electrical characteristics of the transistor 610. Theconductive layer 491 has a function of preventing static electricity. Inthe case where the threshold voltage of the transistor 610 is notrequired to be controlled by the conductive layer 491 in order to makethe transistor 610 be a normally-off transistor, the conductive layer491 is not necessarily provided. In the case where the transistor 610 isused for part of a particular circuit and a problem might be caused byproviding the conductive layer 491, the conductive layer 491 is notnecessarily provided in the circuit.

The wiring layer 692 is electrically connected to the wiring layer 658through an opening in the insulating film 684. In the example in thisembodiment, the insulating film 684 is subjected to planarizationtreatment using a CMP method.

In the semiconductor device, the insulating film 684 is provided betweenthe lower portion and the upper portion, and functions as a bather filmto prevent impurities such as hydrogen, which cause deterioration or achange in electrical characteristics of the transistor 610 in the upperportion, from entering the upper portion from the lower portion.Therefore, a dense inorganic insulating film having a high function ofblocking impurities and the like (e.g., an aluminum oxide film or asilicon nitride film) is preferably used. The insulating film 684 can beformed using the same material as that of the insulating film 433 inEmbodiment 1.

In the case of using the same manufacturing method as that described inEmbodiment 1, the transistor 610 can be manufactured similarly to thetransistor 415. In addition, after the insulating film 407 is formed, aninterlayer insulating film 485 is formed. Further, a semiconductordevice having a multilayer structure in which an embedded wiring isformed in the interlayer insulating film 485 and another semiconductorelement, another wiring, or the like is formed above the embedded wiringmay be manufactured.

In addition, this embodiment can be freely combined with Embodiment 1 orEmbodiment 2.

Embodiment 4

As another example of a semiconductor device including the transistordescribed in Embodiment 1, a cross-sectional view of a NOR circuit,which is a logic circuit, is illustrated in FIG. 5A. FIG. 5B is acircuit diagram of the NOR circuit in FIG. 5A, and FIG. 5C is a circuitdiagram of a NAND circuit.

In the NOR circuit illustrated in FIGS. 5A and 5B, p-channel transistors801 and 802 each have a structure similar to that of the transistor 750in FIGS. 4A and 4B in that a single crystal silicon substrate is usedfor a channel formation region, and n-channel transistors 803 and 804each have a structure similar to that of the transistor 610 in FIGS. 4Aand 4B and that of the transistor 415 in Embodiment 1 in that an oxidesemiconductor film is used for a channel formation region.

In the NOR circuit illustrated in FIGS. 5A and 5B, the transistors 803and 804 each include the conductive layer 491 controlling electricalcharacteristics of the transistors to overlap with a gate electrodelayer with an oxide semiconductor film provided therebetween. Bycontrolling the potential of the conductive layer to GND, for example,the threshold voltages of the transistors 803 and 804 are further movedin the positive direction, so that the transistors can be normally-off.In the NOR circuit in this embodiment, conductive layers which areprovided in the transistors 803 and 804 and can function as back gatesare electrically connected to each other. However, one embodiment of thepresent invention is not limited to this structure, and the conductivelayers functioning as back gates may be individually electricallycontrolled.

In the semiconductor device illustrated in FIG. 5A, a single crystalsilicon substrate is used as a substrate 800, the transistor 802 isformed using the single crystal silicon substrate, and the transistor803 in which stacked oxide semiconductor films are used for a channelformation region is formed over the transistor 802.

The gate electrode layer 401 of the transistor 803 is electricallyconnected to a wiring layer 832. The wiring layer 832 is electricallyconnected to a wiring layer 835. The gate electrode layer 401 of thetransistor 803 is electrically connected to an embedded wiring, and theembedded wiring is electrically connected to an electrode layer 842.Note that the embedded wiring includes a first barrier metal film 486, asecond barrier metal film 488, and a low-resistance conductive layer 487covered by the first barrier metal film 486 and the second barrier metalfilm 488.

The embedded wiring is formed in the following manner. A contact holereaching the electrode layer 842 is formed in the interlayer insulatingfilm 485, the first barrier metal film 486 is formed, and a copper filmor a copper alloy film is formed thereover so as to form thelow-resistance conductive layer 487. Then, polishing is performed forplanarization, and the second barrier metal film 488 is formed so as toprotect the exposed low-resistance conductive layer 487. The embeddedwiring includes the first barrier metal film 486, the second barriermetal film 488, and the low-resistance conductive layer 487 surroundedby the first barrier metal film 486 and the second barrier metal film488.

Each of the first barrier metal film 486 and the second barrier metalfilm 488 may be formed using a conductive material which suppressesdiffusion of copper contained in the low-resistance conductive layer487. Examples of the conductive material are a tantalum nitride film, amolybdenum nitride film, and a tungsten nitride film.

The wiring layer 832 is provided in an opening formed in an insulatingfilm 826 and an insulating film 830, the wiring layer 835 is provided inan opening formed in an insulating film 833, and the electrode layer 842is formed over the wiring layer 835.

An electrode layer 825 of the transistor 802 is electrically connectedto an electrode layer 445 b of the transistor 803, through wiring layers831 and 834. The wiring layer 831 is formed in an opening in theinsulating film 830, the wiring layer 834 is formed in an opening in theinsulating film 833. An electrode layer 445 a and the electrode layer445 b function as source and drain electrode layers of the transistor803.

The first oxide semiconductor film 403 a is formed over and in contactwith the insulating film 437, and the third oxide semiconductor film 403c is formed over and in contact with the second oxide semiconductor film403 b. With the insulating film 437 and the insulating film 402,unnecessary release of oxygen can be suppressed, and the second oxidesemiconductor film 403 b can be kept in an oxygen excess state. Thus, inthe transistor 803, oxygen vacancies in the second oxide semiconductorfilm 403 b and at the interface thereof can be supplied with oxygenefficiently. The transistor 804 has a structure and an effect which aresimilar to those of the transistor 803.

In the NAND circuit in FIG. 5C, p-channel transistors 811 and 814 eachhave a structure similar to that of the transistor 750 in FIGS. 4A and4B, and n-channel transistors 812 and 813 each have a structure similarto that of the transistor 610 in FIGS. 4A and 4B in that an oxidesemiconductor film is used for a channel formation region.

In the NAND circuit illustrated in FIG. 5C, the transistors 812 and 813each includes a conductive layer controlling electrical characteristicsof the transistors to overlap with a gate electrode layer with an oxidesemiconductor film provided therebetween. By controlling the potentialof the conductive layer to GND, for example, the threshold voltages ofthe transistors 812 and 813 are further moved in the positive direction,so that the transistors can be normally-off. In the NAND circuit in thisembodiment, conductive layers which are provided in the transistors 812and 813 and function as back gates are electrically connected to eachother. However, one embodiment of the present invention is not limitedto this structure, and the conductive layers functioning as back gatesmay be individually electrically controlled.

In the semiconductor device in this embodiment, the transistor in whichan oxide semiconductor is used for the channel formation region andwhich has extremely low off-state current is employed; therefore, powerconsumption can be sufficiently reduced.

Further, with a stack of semiconductor elements using differentsemiconductor materials, a miniaturized and highly integratedsemiconductor device with stable electric characteristics and a methodfor manufacturing the semiconductor device can be provided.

The NOR circuit and the NAND circuit including the transistors describedin Embodiment 1 are described as examples in this embodiment; however,the present invention is not limited to the circuits, and an ANDcircuit, an OR circuit, or the like can be formed using the transistorsdescribed in Embodiment 1 or 2. For example, a semiconductor device(storage device) in which stored data can be held even when power is notsupplied and which has an unlimited number of times of writing with thetransistors described in Embodiment 1 or 2 can be manufactured.

FIG. 6 is an example of a circuit diagram of a semiconductor device.

In FIG. 6, a first wiring (1st line) is electrically connected to asource electrode layer of a transistor 160. A second wiring (2nd line)is electrically connected to a drain electrode layer of the transistor160. Any of the transistors 740, 750, and 802 described in thisembodiment can be used as the transistor 160.

A third wiring (3rd line) is electrically connected to one of a sourceelectrode layer and a drain electrode layer of a transistor 162, and afourth line (4th line) is electrically connected to a gate electrodelayer of the transistor 162. A gate electrode layer of the transistor160 and the other of the source electrode layer and the drain electrodelayer of the transistor 162 are electrically connected to one electrodeof a capacitor 164. A fifth wiring (5th line) and the other electrode ofthe capacitor 164 are electrically connected to each other.

Any one of the transistors 415, 416, and 417 described in Embodiment 1or 2 can be used as the transistor 162.

A semiconductor device having the circuit configuration in FIG. 6utilizes a characteristic in which the potential of the gate electrodelayer of the transistor 160 can be held, and thus enables data writing,holding, and reading as follows.

Writing and holding of data will be described. First, the potential ofthe fourth line is set to a potential at which the transistor 162 isturned on, so that the transistor 162 is turned on. Accordingly, thepotential of the third wiring is supplied to the gate electrode layer ofthe transistor 160 and to the capacitor 164. That is, predeterminedcharge is supplied to the gate electrode layer of the transistor 160(writing). Here, charge for supply of a potential level or charge forsupply of a different potential level (hereinafter referred to as Lowlevel charge and High level charge) is given. After that, the potentialof the fourth wiring is set to a potential at which the transistor 162is turned off, so that the transistor 162 is turned off. Thus, thecharge given to the gate electrode layer of the transistor 160 is held(holding).

Since the amount of off-state current of the transistor 162 is extremelysmall, the charge of the gate electrode layer of the transistor 160 isheld for a long time.

Next, reading of data will be described. By supplying an appropriatepotential (a reading potential) to the fifth wiring while supplying apredetermined potential (a constant potential) to the first wiring, thepotential of the second wiring varies depending on the amount of chargeheld at the gate electrode layer of the transistor 160. This is becausein general, when the transistor 160 is an n-channel transistor, anapparent threshold voltage V_(th) _(—) _(H) in the case where thehigh-level potential is given to the gate electrode layer of thetransistor 160 is lower than an apparent threshold voltage V_(th) _(—)_(L) in the case where the low-level charge is given to the gateelectrode layer of the transistor 160. Here, an apparent thresholdvoltage refers to the potential of the fifth line, which is needed toturn on the transistor 160. Thus, the potential of the fifth wiring isset to a potential V₀ that is between V_(th) _(—) _(H) and V_(th) _(—)_(L), whereby charge supplied to the gate electrode layer of thetransistor 160 can be determined. For example, in the case where ahigh-level charge is given in writing, when the potential of the fifthwiring is set to V₀ (>V_(th) _(—) _(H)), the transistor 160 is turnedon. In the case where a low level charge is given in writing, even whenthe potential of the fifth wiring is set to V₀ (<V_(th) _(—) _(L)), thetransistor 160 remains in an off state. Therefore, the stored data canbe read by the potential of the second line.

Note that in the case where memory cells are arrayed to be used, onlydata of desired memory cells needs to be read. In the memory cell wheredata is not read, a potential at which the transistor 160 is turned offregardless of the state of the gate electrode layer of the transistor160, that is, a potential smaller than V_(th) _(th) _(—) _(H) may begiven to the fifth wiring. Alternatively, a potential at which thetransistor 160 is turned on regardless of the state of the gateelectrode layer, that is, a potential higher than V_(th) _(th) _(—) _(L)may be given to the fifth line.

FIG. 7 illustrates an example of one embodiment of a structure of amemory device.

FIG. 7 is a perspective view of a memory device. The memory deviceillustrated in FIG. 7 includes a plurality of layers of memory cellarrays (memory cell arrays 3400_1 to 3400 _(—) n (n is an integergreater than or equal to 2)) each including a plurality of memory cellsas memory circuits in the upper portion, and a logic circuit 3004 in thelower portion which is necessary for operating the memory cell arrays3400_1 to 3400 _(—) n.

FIG. 7 illustrates the logic circuit 3004, the memory cell array 3400_1,and the memory cell array 3400_2, and illustrates a memory cell 3170 aand a memory cell 3170 b as typical examples in the plurality of memorycells included in the memory cell array 3400_1 and the memory cell array3400_2. The memory cell 3170 a and the memory cell 3170 b can have aconfiguration similar to the circuit configuration described in thisembodiment with reference to FIG. 6, for example.

Note that as transistors included in the memory cells 3170 a and 3170 b,a transistor in which a channel formation region is formed in an oxidesemiconductor film is used. The structure of the transistor in which thechannel formation region is formed in the oxide semiconductor film isthe same as the structure described in Embodiment 1, and thus thedescription of the structure is omitted.

The logic circuit 3004 includes a transistor in which a semiconductormaterial other than an oxide semiconductor is used for a channelformation region. For example, a transistor obtained by providing anelement isolation insulating layer on a substrate containing asemiconductor material (e.g., silicon) and forming a region serving asthe channel formation region in a region surrounded by the elementisolation insulating layer can be used. Note that the transistor may bea transistor obtained in such a manner that the channel formation regionis formed in a semiconductor film such as a polycrystalline silicon filmformed on an insulating surface or in a silicon film of an SOIsubstrate.

The memory cell arrays 3400_1 to 3400 _(—) n and the logic circuit 3004are stacked with interlayer insulating layers provided therebetween, andare electrically connected to each other as appropriate throughelectrodes and wirings which penetrate the interlayer insulating layers,for example.

When a transistor having a channel formation region formed using anoxide semiconductor and having extremely low off-state current isemployed in the semiconductor device in this embodiment, thesemiconductor device can store data for an extremely long period. Inother words, power consumption can be adequately reduced because refreshoperation becomes unnecessary or the frequency of refresh operation canbe extremely low. Moreover, stored data can be held for a long periodeven when power is not supplied (note that a potential is preferablyfixed).

Further, in the semiconductor device described in this embodiment, highvoltage is not needed for writing data and there is no problem ofdeterioration of elements. For example, unlike a conventionalnonvolatile memory, it is not necessary to inject and extract electronsinto and from a floating gate; thus, the problem of deterioration of agate insulating film does not occur. In other words, the semiconductordevice according to one embodiment of the present invention does nothave a limit on the number of times of writing which is a problem in aconventional nonvolatile memory, and reliability thereof is drasticallyimproved. Furthermore, data is written depending on the on state and theoff state of the transistor, whereby high-speed operation can be easilyachieved.

As described above, a miniaturized and highly-integrated semiconductordevice having high electric characteristics and a method formanufacturing the semiconductor device can be provided.

This embodiment can be freely combined with any of Embodiment 1,

Embodiment 2, and Embodiment 3.

Embodiment 5

In this embodiment, a central processing unit (CPU) at least part ofwhich includes any of the transistors 415, 416, and 417 described inEmbodiment 1 or 2 is described as an example of a semiconductor device.

FIG. 8A is a block diagram illustrating a specific structure of the CPU.The CPU illustrated in FIG. 8A includes an arithmetic logic unit (ALU)1191, an ALU controller 1192, an instruction decoder 1193, an interruptcontroller 1194, a timing controller 1195, a register 1196, a registercontroller 1197, a bus interface (Bus I/F) 1198, a rewritable ROM 1199,and an ROM interface (ROM I/F) 1189 over a substrate 1190. Asemiconductor substrate, an SOI substrate, a glass substrate, or thelike is used as the substrate 1190. The ROM 1199 and the ROM interface1189 may each be provided over a separate chip. Obviously, the CPUillustrated in FIG. 8A is just an example in which a configuration issimplified, and actual CPUs may have a variety of configurationsdepending on the application.

An instruction that is input to the CPU through the bus interface 1198is input to the instruction decoder 1193 and decoded therein, and then,input to the ALU controller 1192, the interrupt controller 1194, theregister controller 1197, and the timing controller 1195.

The ALU controller 1192, the interrupt controller 1194, the registercontroller 1197, and the timing controller 1195 conduct various controlsin accordance with the decoded instruction. Specifically, the ALUcontroller 1192 generates signals for controlling the operation of theALU 1191. While the CPU is executing a program, the interrupt controller1194 judges an interrupt request from an external input/output device ora peripheral circuit on the basis of its priority or a mask state, andprocesses the request. The register controller 1197 generates an addressof the register 1196, and reads/writes data from/to the register 1196 inaccordance with the state of the CPU.

The timing controller 1195 generates signals for controlling operationtimings of the ALU 1191, the ALU controller 1192, the instructiondecoder 1193, the interrupt controller 1194, and the register controller1197. For example, the timing controller 1195 includes an internal clockgenerator for generating an internal clock signal CLK2 based on areference clock signal CLK1, and supplies the internal clock signal CLK2to the above circuits.

In the CPU illustrated in FIG. 8A, a memory cell is provided in theregister 1196. As the memory cell of the register 1196, the memory celldescribed in Embodiment 4 can be used.

In the CPU illustrated in FIG. 8A, the register controller 1197 selectsan operation of holding data in the register 1196 in accordance with aninstruction from the ALU 1191. That is, the register controller 1197selects whether data is held by a flip-flop or by a capacitor in thememory cell included in the register 1196. When data holding by theflip-flop is selected, a power supply voltage is supplied to the memorycell in the register 1196. When data holding by the capacitor isselected, the data is rewritten in the capacitor, and supply of powersupply voltage to the memory cell in the register 1196 can be stopped.

The supply of power can be stopped with a switching element providedbetween a memory cell group and a node to which a power supply potentialVDD or a power supply potential VSS is supplied, as illustrated in FIG.8B or FIG. 8C. Circuits illustrated in FIGS. 8B and 8C are describedbelow.

FIGS. 8B and 8C each illustrate an example of a memory circuitconfiguration in which any one of transistors 415, 416, and 417 inEmbodiment 1 or 2 is used as a switching element which controls a supplyof a power supply potential to a memory cell.

The memory device illustrated in FIG. 8B includes a switching element1141 and a memory cell group 1143 including a plurality of memory cells1142. Specifically, as each of the memory cells 1142, the memory celldescribed in Embodiment 3 can be used. Each of the memory cells 1142included in the memory cell group 1143 is supplied with the high-levelpower supply potential VDD via the switching element 1141. Further, eachof the memory cells 1142 included in the memory cell group 1143 issupplied with a potential of a signal IN and the low-level power supplypotential VSS.

In FIG. 8B, any one of the transistors 415, 416, and 417 described inEmbodiment 1 or 2 is used as the switching element 1141, and theswitching of the transistor is controlled by a signal Sig A supplied toa gate electrode layer thereof.

Note that FIG. 8B illustrates the structure in which the switchingelement 1141 includes only one transistor; however, the structure is notlimited thereto, and the switching element 1141 may include a pluralityof transistors. In the case where the switching element 1141 includes aplurality of transistors which serves as switching elements, theplurality of transistors may be connected to each other in parallel, inseries, or in combination of parallel connection and series connection.

Although the switching element 1141 controls the supply of thehigh-level power supply potential VDD to each of the memory cells 1142included in the memory cell group 1143 in FIG. 8B, the switching element1141 may control the supply of the low-level power supply potential VSS.

In FIG. 8C, an example of a memory device in which each of the memorycells 1142 included in the memory cell group 1143 is supplied with thelow-level power supply potential VSS via the switching element 1141 isillustrated. The supply of the low-level power supply potential VSS toeach of the memory cells 1142 included in the memory cell group 1143 canbe controlled by the switching element 1141.

When a switching element is provided between a memory cell group and anode to which the power supply potential VDD or the power supplypotential VSS is supplied, data can be held even in the case where anoperation of a CPU is temporarily stopped and the supply of the powersupply voltage is stopped; accordingly, power consumption can bereduced. Specifically, for example, while a user of a personal computerdoes not input data to an input device such as a keyboard, the operationof the CPU can be stopped, so that the power consumption can be reduced.

Although the CPU is given as an example, the transistor can also beapplied to an LSI such as a digital signal processor (DSP), a customLSI, or a field programmable gate array (FPGA).

The methods and structures described in this embodiment can be combinedas appropriate with any of the methods and structures described in theother embodiments.

Embodiment 6

A semiconductor device disclosed in this specification can be applied toa variety of electronic devices (including game machines). Examples ofthe electronic devices include display devices of televisions, monitors,and the like, lighting devices, desktop personal computers and laptoppersonal computers, word processors, image reproduction devices whichreproduce still images or moving images stored in recording media suchas digital versatile discs (DVDs), portable compact disc (CD) players,radio receivers, tape recorders, headphone stereos, stereos, cordlessphone handsets, transceivers, portable wireless devices, mobile phones,car phones, portable game machines, calculators, portable informationterminals, electronic notebooks, e-book readers, electronic translators,audio input devices, cameras such as still cameras and video cameras,electric shavers, high-frequency heating appliances such as microwaveovens, electric rice cookers, electric washing machines, electric vacuumcleaners, air-conditioning systems such as air conditioners,dishwashers, dish dryers, clothes dryers, futon dryers, electricrefrigerators, electric freezers, electric refrigerator-freezers,freezers for preserving DNA, smoke detectors, radiation counters, andmedical equipment such as dialyzers. Further, the examples includeindustrial equipment such as guide lights, traffic lights, beltconveyors, elevators, escalators, industrial robots, and power storagesystems. In addition, oil engines, moving objects driven by electricmotors using power from the non-aqueous secondary batteries, and thelike are also included in the category of electronic devices. Examplesof the moving objects include electric vehicles (EV), hybrid electricvehicles (HEV) which include both an internal-combustion engine and amotor, plug-in hybrid electric vehicles (PHEV), tracked vehicles inwhich caterpillar tracks are substituted for wheels of these vehicles,motorized bicycles including motor-assisted bicycles, motorcycles,electric wheelchairs, golf carts, boats or ships, submarines,helicopters, aircrafts, rockets, artificial satellites, space probes,planetary probes, spacecrafts, and the like. Specific examples of suchelectronic devices are illustrated in FIGS. 9A to 9C and FIGS. 10A to10C.

FIGS. 9A and 9B illustrate a tablet terminal that can be folded. In FIG.9A, the tablet terminal is opened, and includes a housing 9630, adisplay portion 9631 a, a display portion 9631 b, a display-modeswitching button 9034, a power button 9035, a power-saving-modeswitching button 9036, a clip 9033, and an operation button 9038.

In such a portable device illustrated in FIGS. 9A and 9B, an SRAM or aDRAM is used as a memory element for temporarily storing image data. Forexample, the semiconductor device described in Embodiment 4 can be usedas a memory. By employing the semiconductor device described in theabove embodiment for the memory, data can be written and read at highspeed and held for a long time, and power consumption can besufficiently reduced. A CPU for performing image processing orarithmetic processing is used in the portable device illustrated inFIGS. 9A and 9B. As the CPU, the CPU described in Embodiment 5 can beused, in which case the CPU described in Embodiment 5 is used, powerconsumption of the portable device can be reduced.

A touch panel region 9632 a can be provided in a part of the displayportion 9631 a, in which data can be input by touching displayedoperation keys 9638. Although half of the display portion 9631 a hasonly a display function and the other half has a touch panel function,one embodiment of the present invention is not limited to the structure.However, the structure of the display portion 9631 a is not limited tothis, and all the area of the display portion 9631 a may have a touchpanel function. For example, all the area of the display portion 9631 acan display keyboard buttons and serve as a touch panel while thedisplay portion 9631 b can be used as a display screen.

Like the display portion 9631 a, part of the display portion 9631 b canbe a touch panel region 9632 b. When a finger, a stylus, or the liketouches the place where a button 9639 for switching to keyboard displayis displayed in the touch panel, keyboard buttons can be displayed onthe display portion 9631 b.

Touch input can be performed concurrently on the touch panel regions9632 a and 9632 b.

The display-mode switching button 9034 allows switching between alandscape mode and a portrait mode, color display and black-and-whitedisplay, and the like. With the switching button 9036 for switching topower-saving mode, the luminance of display can be optimized inaccordance with the amount of external light at the time when the tabletis in use, which is detected with an optical sensor incorporated in thetablet. The tablet may include another detection device such as a sensorfor detecting orientation (e.g., a gyroscope or an acceleration sensor)in addition to the optical sensor.

Although the display portion 9631 a and the display portion 9631 b havethe same display size in FIG. 9A, an embodiment of the present inventionis not limited to this example. The display portion 9631 a and thedisplay portion 9631 b may have different sizes or different displayquality. For example, one of them may be a display panel that candisplay higher-definition images than the other.

FIG. 9B illustrates the tablet terminal folded, which includes thehousing 9630, a solar cell 9633, a charge and discharge control circuit9634, a battery 9635, and a DCDC converter 9636. Note that FIG. 9Billustrates an example in which the charge and discharge control circuit9634 includes the battery 9635 and the DCDC converter 9636.

Since the tablet can be folded in two, the housing 9630 can be closedwhen the tablet is not in use. Thus, the display portions 9631 a and9631 b can be protected, and thereby providing a tablet with highendurance and high reliability for long-term use.

The tablet terminal illustrated in FIGS. 9A and 9B can have otherfunctions such as a function of displaying various kinds of data (e.g.,a still image, a moving image, and a text image), a function ofdisplaying a calendar, a date, the time, or the like on the displayportion, a touch-input function of operating or editing the datadisplayed on the display portion by touch input, and a function ofcontrolling processing by various kinds of software (programs).

The solar cell 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch panel, a display portion,an image signal processor, and the like. Note that the solar cell 9633can be provided on one or both surfaces of the housing 9630, so that thebattery 9635 can be charged efficiently. When a lithium ion battery isused as the battery 9635, there is an advantage of downsizing or thelike.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 9B are described with reference to a blockdiagram of FIG. 9C. FIG. 9C illustrates the solar cell 9633, the battery9635, the DCDC converter 9636, a converter 9637, switches SW1 to SW3,and the display portion 9631. The battery 9635, the DCDC converter 9636,the converter 9637, and the switches SW1 to SW3 correspond to the chargeand discharge control circuit 9634 in FIG. 9B.

First, an example of operation in the case where power is generated bythe solar cell 9633 using external light is described. The voltage ofpower generated by the solar cell 9633 is raised or lowered by the DCDCconverter 9636 so that a voltage for charging the battery 9635 isobtained. When the display portion 9631 is operated with the power fromthe solar cell 9633, the switch SW1 is turned on and the voltage of thepower is raised or lowered by the converter 9637 to a voltage needed foroperating the display portion 9631. In addition, when display on thedisplay portion 9631 is not performed, the switch SW1 is turned off anda switch SW2 is turned on so that charge of the battery 9635 may beperformed.

Here, the solar cell 9633 is shown as an example of a power generationmeans; however, there is no particular limitation on a way of chargingthe battery 9635, and the battery 9635 may be charged with another powergeneration means such as a piezoelectric element or a thermoelectricconversion element (Peltier element). For example, the battery 9635 maybe charged with a non-contact power transmission module that transmitsand receives power wirelessly (without contact) to charge the battery orwith a combination of other charging means.

In a television set 8000 in FIG. 10A, a display portion 8002 isincorporated in a housing 8001. The display portion 8002 displays animage and a speaker portion 8003 can output sound.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoresis displaydevice, a digital micromirror device (DMD), or a plasma display panel(PDP) can be used for the display portion 8002.

The television set 8000 may be provided with a receiver, a modem, andthe like. Furthermore, when the television set 8000 is connected to acommunication network by wired or wireless connection via the modem,one-way (from a transmitter to a receiver) or two-way (between atransmitter and a receiver, between receivers, or the like) datacommunication can be performed.

In addition, the television set 8000 may include a CPU for performinginformation communication or a memory. The memory described inEmbodiment 4 or the CPU described in Embodiment 5 can be used in thetelevision set 8000.

In FIG. 10A, an air conditioner which includes an indoor unit 8200 andan outdoor unit 8204 is an example of an electronic device in which theCPU of Embodiment 5 is used. Specifically, the indoor unit 8200 includesa housing 8201, an air outlet 8202, a CPU 8203, and the like. Althoughthe CPU 8203 is provided in the indoor unit 8200 in FIG. 10A, the CPU8203 may be provided in the outdoor unit 8204. Alternatively, the CPU8203 may be provided in both the indoor unit 8200 and the outdoor unit8204. By using the CPU described in Embodiment 5 as the CPU in the airconditioner, power consumption can be reduced.

In FIG. 10A, an electric refrigerator-freezer 8300 is an example of anelectronic device which is provided with the CPU formed using an oxidesemiconductor. Specifically, the electric refrigerator-freezer 8300includes a housing 8301, a door for a refrigerator 8302, a door for afreezer 8303, a CPU 8304, and the like. In FIG. 10A, the CPU 8304 isprovided in the housing 8301. When the CPU described in Embodiment 5 isused as the CPU 8304 of the electric refrigerator-freezer 8300, powerconsumption of the electric refrigerator-freezer 8300 can be reduced.

FIG. 10B illustrates an example of an electric vehicle which is anexample of an electronic device. An electric vehicle 9700 is equippedwith a secondary battery 9701 (FIG. 10C). The output of the electricpower of the secondary battery 9701 is adjusted by a control circuit9702 so that the electric power is supplied to a driving device 9703.The control circuit 9702 is controlled by a processing unit 9704including a ROM, a RAM, a CPU, or the like which is not illustrated.When the CPU described in Embodiment 5 is used as the CPU in theelectric vehicle 9700, power consumption of the electric vehicle 9700can be reduced.

The driving device 9703 includes a DC motor or an AC motor either aloneor in combination with an internal-combustion engine. The processingunit 9704 outputs a control signal to the control circuit 9702 based oninput data such as data of operation (e.g., acceleration, deceleration,or stop) by a driver or data during driving (e.g., data on an upgrade ora downgrade, or data on a load on a driving wheel) of the electricvehicle 9700. The control circuit 9702 adjusts the electric energysupplied from the secondary battery 9701 in accordance with the controlsignal of the processing unit 9704 to control the output of the drivingdevice 9703. In the case where the AC motor is mounted, although notillustrated, an inverter which converts direct current into alternatecurrent is also incorporated.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

EXPLANATION OF REFERENCE

-   160: transistor, 162: transistor, 164: capacitor, 400: substrate,    401: gate electrode layer, 402: insulating film, 403: stacked layer    including oxide semiconductor films, 403 a: first oxide    semiconductor film, 403 b: second oxide semiconductor film, 403 c:    third oxide semiconductor film, 415: transistor, 416: transistor,    417: transistor, 433: insulating film, 434: wiring layer, 435: oxide    insulating film, 436: wiring layer, 437: insulating film, 438:    electrode layer, 439: interlayer insulating film, 442: conductive    layer, 445 a: electrode layer, 445 b: electrode layer, 485:    interlayer insulating film, 486: barrier metal film, 487:    low-resistance conductive layer, 488: barrier metal film, 491:    conductive layer, 610: transistor, 647: wiring layer, 657: wiring    layer, 658: wiring layer, 684: insulating film, 686: insulating    film, 687: insulating film, 690: capacitor, 692: wiring layer, 693:    capacitor electrode layer, 700: substrate, 740: transistor, 741:    gate electrode layer, 742: gate insulating film, 743: channel    formation region, 744: n-type impurity region, 745: n-type impurity    region, 746: sidewall insulating layer, 748: wiring layer, 750:    transistor, 751: gate electrode layer, 752: gate insulating film,    753: channel formation region, 754: p-type impurity region, 755:    p-type impurity region, 756: sidewall insulating layer, 760:    circuit, 788: insulating film, 789: element separation region, 800:    substrate, 801: transistor, 802: transistor, 803: transistor, 804:    transistor, 811: transistor, 812: transistor, 813: transistor, 814:    transistor, 825: electrode layer, 826: insulating film, 830:    insulating film, 831: wiring layer, 832: wiring layer, 833:    insulating film, 834: wiring layer, 835: wiring layer, 842:    electrode layer, 1141: switching element, 1142: memory cell, 1143:    memory cell group, 1189: ROM interface, 1190: substrate, 1191: ALU,    1192: ALU controller, 1193: instruction decoder, 1194: interrupt    controller, 1195: timing controller, 1196: register, 1197: register    controller, 1198: bus interface, 1199: ROM, 3004: logic circuit,    3170 a: memory cell, 3170 b: memory cell, 3400: memory cell array,    8000: television set, 8001: housing, 8002: display portion, 8003:    speaker portion, 8200: indoor unit, 8201: housing, 8202: air outlet,    8203: CPU, 8204: outdoor unit, 8300: electric refrigerator-freezer,    8301: housing, 8302: door for refrigerator, 8303: door for freezer,    8304: CPU, 9033: clip, 9034: switching button, 9035: power button,    9036: switching button, 9038: operation button, 9630: housing, 9631:    display portion, 9631 a: display portion, 9631 b: display portion,    9632 a: region, 9632 b: region, 9633: solar cell, 9634: charge and    discharge control circuit, 9635: battery, 9636: DCDC converter,    9637: converter, 9638: operation key, 9639: button, 9700: electric    vehicle, 9701: secondary battery, 9702: control circuit, 9703:    driving device, 9704: processing unit

This application is based on Japanese Patent Application serial no.2012-125394 filed with Japan Patent Office on May 31, 2012, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A semiconductor device comprising: a first buffer layer over an insulating layer; an oxide semiconductor layer over the first buffer layer; and a second buffer layer over the oxide semiconductor layer, wherein each of the oxide semiconductor layer, the first buffer layer, and the second buffer layer comprises indium and gallium, wherein a composition of indium with respect to gallium in the oxide semiconductor layer is higher than a composition of indium with respect to gallium in the first buffer layer, and wherein the composition of indium with respect to gallium in the oxide semiconductor layer is higher than a composition of indium with respect to gallium in the second buffer layer.
 3. The semiconductor device according to claim 2, wherein the oxide semiconductor layer comprises crystals whose c-axes are aligned in a direction substantially perpendicular to a formation surface or a top surface of the oxide semiconductor layer.
 4. The semiconductor device according to claim 2, wherein a carrier flows at an interface between the first buffer layer and the oxide semiconductor layer or an interface between the second buffer layer and the oxide semiconductor layer.
 5. The semiconductor device according to claim 2, wherein a carrier flows in the oxide semiconductor layer.
 6. The semiconductor device according to claim 5, wherein the oxide semiconductor layer is not in contact with the insulating layer as a gate insulating layer.
 7. The semiconductor device according to claim 2, wherein a thickness of the first buffer layer or the second buffer layer is greater than or equal to 2 nm and less than or equal to 15 nm.
 8. A semiconductor device comprising: a first gate electrode; a first gate insulating layer over the first gate electrode; a first oxide semiconductor layer over the first gate insulating layer; a second oxide semiconductor layer over the first oxide semiconductor layer; a third oxide semiconductor layer over the second oxide semiconductor layer; a second gate insulating layer over the third oxide semiconductor layer; and a second gate electrode over the second gate insulating layer, wherein a composition of indium with respect to gallium in the second oxide semiconductor layer is higher than a composition of indium with respect to gallium in the first oxide semiconductor layer, and wherein the composition of indium with respect to gallium in the second oxide semiconductor layer is higher than a composition of indium with respect to gallium in the third oxide semiconductor layer.
 9. The semiconductor device according to claim 8, wherein the second oxide semiconductor layer comprises crystals whose c-axes are aligned in a direction substantially perpendicular to a formation surface or a top surface of the second oxide semiconductor layer.
 10. The semiconductor device according to claim 8, wherein a carrier flows at an interface between the first oxide semiconductor layer and the second oxide semiconductor layer or an interface between the third oxide semiconductor layer and the second oxide semiconductor layer.
 11. The semiconductor device according to claim 8, wherein a carrier flows in the second oxide semiconductor layer.
 12. The semiconductor device according to claim 11, wherein the second oxide semiconductor layer is not in contact with the first gate insulating layer or the second gate insulating layer.
 13. The semiconductor device according to claim 8, wherein a thickness of the first oxide semiconductor layer or the third oxide semiconductor layer is greater than or equal to 2 nm and less than or equal to 15 nm.
 14. A semiconductor device comprising: a first gate electrode; a first gate insulating layer over the first gate electrode; a first oxide semiconductor layer over the first gate insulating layer; a second oxide semiconductor layer over the first oxide semiconductor layer; a third oxide semiconductor layer over the second oxide semiconductor layer; a second gate insulating layer over the third oxide semiconductor layer; and a second gate electrode over the second gate insulating layer, wherein an energy gap of the third oxide semiconductor layer is smaller than an energy gap of the first oxide semiconductor layer, and wherein the energy gap of the third oxide semiconductor layer is smaller than an energy gap of the second oxide semiconductor layer.
 15. The semiconductor device according to claim 14, wherein the second oxide semiconductor layer comprises crystals whose c-axes are aligned in a direction substantially perpendicular to a formation surface or a top surface of the second oxide semiconductor layer.
 16. The semiconductor device according to claim 14, wherein a carrier flows at an interface between the first oxide semiconductor layer and the second oxide semiconductor layer or an interface between the third oxide semiconductor layer and the second oxide semiconductor layer.
 17. The semiconductor device according to claim 14, wherein a carrier flows in the second oxide semiconductor layer.
 18. The semiconductor device according to claim 17, wherein the second oxide semiconductor layer is not in contact with the first gate insulating layer or the second gate insulating layer.
 19. The semiconductor device according to claim 14, wherein a thickness of the first oxide semiconductor layer or the third oxide semiconductor layer is greater than or equal to 2 nm and less than or equal to 15 nm. 